A prior art access network using copper wires employs, for example, a digital subscriber line (xDSL, where x refers to different variants of DSL, such as ADSL, VDSL, etc.) using telephone lines and a cable modem technology, etc. using a coaxial cable. This prior art access network has a disadvantage that a bandwidth may be limited depending on a transmission distance. Thus, only one single method to overcome the limit of a transmission distance is to use a single mode fiber (SMF) which provides an unlimited transmission bandwidth. It is referred to a fiber-to-the-home (FTTH) method where an SMF is used as a transmission medium and is installed to subscribers so that information is given and taken therethrough. In this case, an FTTH is especially referred to a passive optical network (PON) where the FTTH uses a point-to-multipoint architecture to reduce the number of an SMF necessary for installation from a central office (CO) to subscribers, while being only comprised of passive elements without using active elements/systems.
A PON technology is mainly classified as a TDM-PON and a WDM-PON depending on a method of sharing an optical fiber, and the TDM-PON is referred to a PON which shares one optical fiber using a time division multiple access (TDMA).
The TDM-PON started to be commercialized as a need for the FTTH is required. As specific examples of the TDM-PON, there had been an asynchronous transfer mode (ATM)-PON or a Broadband-PON (hereinafter referred to ‘B-PON’, while an Ethernet-PON (hereinafter referred to ‘E-PON’ having a transmission speed of 1 Gb/s was commercialized in the year of early 2000 (See K. Ohara, et al., “traffic analysis of Ethernet-PON in FTTH trial service, Optical Fiber Comm. Technical Digest, Anaheim, Calif., pp. 607-608, March 2003).
After that, a Gigabit-PON (hereinafter referred to ‘G-PON’) capable of transmitting signals with a transmission speed of 2.5 Gb/s has been developed, and is now at a stage of commercialization.
However, in a TDM-PON, upstream and downstream transmission speeds are fixed at constant standard speeds depending on the kinds of the PON and a plurality of subscribers shares the TDM-PON commonly so that a bandwidth per each subscriber to be provided for is reduced when expanding the number of subscribers (i.e., when increasing a split ratio). For example, in a TDM-PON having 32 split ratio, upstream and downstream transmission speeds are approximately 30 Mb/s, respectively, as an average bandwidth per each subscriber to be provided for in case of E-PON (where both of upstream and downstream transmission speeds are 1.25 Gb/s), while upstream and downstream transmission speeds are approximately 36 Mb/s and 72 Mb/s, respectively, as an average bandwidth per each subscriber to be provided for in case of G-PON (where a downstream transmission speed is 2.5 Gb/s and an upstream transmission speed is 1.25 Gb/s). Further, although a higher speed of TDM-PON is required due to the rapid increase of demand for broadband services as the use of internet is increased and image and video services are widely spread, there are many technical problems to be solved to accomplish a higher speed of TDM-PON. Accordingly, a next-generation PON capable of providing broadband service at a low cost is actively discussed by a standard group such as FSAN (Full Service Access Network) or IEEE. Here, one of the requirements for next-generation PON is to use an optical fiber and a splitter which are an already installed infrastructure for a legacy TDM-PON.